Unit 3.2 : Redox Reactions Flashcards
How are redox titrations carried out?
In the same way as acid-base titrations
What’s difference between acid-base titrations and redox titrations?
Redox titrations involve redox reactions
2 most important redox reagents in titrations
Potassium manganate (VII)
Sodium thiosulfate
Which important redox reagent isn’t used much in titrations anymore and why?
Potassium dichromate (VI), it’s carsinogenical
Half equation for the reduction of manganate (VII) ions in acidic solution
MnO4- + 8H+ + 5e- —> Mn2+ + 4H2O
What can manganate VII be described as?
An oxidising agent
What is potassium manganate (VII) used for in redox titrations?
To estimate the concentration of iron (II) ions in aqueous solution
Half equation for the oxidation of iron (II) ions
Fe2+ —> Fe3+ + e-
How do we form a full redox equation from half equations?
Balance the electrons
Full redox equation of potassium manganate (VII) and iron (II) ions
MnO4- + 5Fe2+ + 8H+ ——> Mn2+ + 5Fe3+ + 4H2O
Ratio of the number of moles of MnO4 to the number of moles of Fe2+
1:5
How is the titration between potassium manganate (VII) and iron (II) ions set up?
Potassium manganate (VII) in the burette, iron (II) complex in flask below
What’s the end point of the redox titration between potassium manganate (VII) and iron (II) ions?
Persisting pale pink
Briefly explain how the redox titration between potassium manganate (VII) and iron (II) ions would be completed
The potassium manganate (VII) solution is added from a burette to the iron (II) solution in the flask and the reaction is complete when the 1st permanent pink tinge is seen
What is the indicator in the redox reaction between potassium manganate (VII) and iron (II) ions?
There isn’t one - it’s self indicating
Why does potassium manganate (VII) not need an indicator?
Self-indicating
What colour does potassium manganate (VII) change to and from when the redox titration is complete?
Colourless to purple
What needs to be done to potassium manganate (VII) for a redox reaction to proceed satisfactorily?
Must be acidified
What do iron (II) do in air?
Readily oxidise
What needs to be done as iron (II) ions readily oxidise in air?
The solutions need to be acidified with dilute sulfuric acid to prevent oxidation
How does acid force the redox reaction to complete when added to the solution?
Removed the oxygen from the oxidising agent
What is the oxidising agent in the redox titration between potassium manganate and iron (II) ions?
Manganate (VII) ions
How much sulfuric acid are iron (II) ions acidified with to prevent oxidation?
1moldm-3
Why is sulfuric acid chosen as the acid to acidify potassium manganate (VII)?
It will not react with the manganate (VII) ions
Is potassium manganate (VII) an oxidising or a reducing agent?
An extremely powerful oxidising agent
How powerful of an oxidising agent is potassium manganate (VII)?
The most powerful oxidising agent which is reasonably stable in aqueous solution
What does potassium manganate do slowly in aqueous solution?
Decompose
What needs to happen to potassium manganate before it can be used in a redox titration reaction?
Needs to be standardised
What is usually used to standardise potassium manganate (VII) solution
Ammonium iron (II) sulfate
Why is ammonium iron (II) sulfate normally used to standardise potassium manganate (VII)?
It does not oxidise readily in solution and can be obtained to a high degree of purity
Why can’t iron (II) salts be used to standardise potassium manganate?
They cannot normal be obtained to a high degree of purity
Why would a brown precipitate of manganese (IV) oxide be formed during titration?
A neutral/alkaline solution must have been used
What would using a neutral/alkaline solution during the titration of potassium manganate (VII) result in
A brown precipitate of manganese (IV) oxide as a different reaction as taken place
What is the concentration of potassium manganate (VII) normally used during its redox titration?
0.02moldm-3
What does the concentration of the iron (II) ions need to be in it’s redox titration and how is this worked out?
About 0.100moldm-3 as the concentration of potassium manganate (VII) normally used is 0.02moldm-3 and they’re in a 1:5 ratio
What would adding iron (II) ions of concentration 0.100moldm-3 to potassium manganate (VII) of concentration 0.02moldm-3 do?
Complete reaction after the addition of 25cm3 of manganate (VII) solution to 25cm3 of the iron (II) solution
What tend to be self indicating in titrations?
Transition metals
How do we know exactly how much of a salt has been added in a reaction?
Mass of solid
Mass of solid + container
Mass of container
What happens to the solution that’s left unreacted at the end of a reaction?
It’s still present
Dichromate ions
Cr2O7^2-
Oxyanions
Negative ions that contain oxygen atoms bonded to another element
Negative ions that contain oxygen atoms bonded to another element
Oxyanions
Cr2O7^2- ion
Dichromate
Under which conditions can half equations for Oxyanions occur?
Either acidic or alkaline